Semiconductor device

ABSTRACT

Provided is a semiconductor device exemplified by an inverter circuit and a shift register circuit, which is characterized by a reduced number of transistors. The semiconductor device includes a first transistor, a second transistor, and a capacitor. One of a source and a drain of the first transistor is electrically connected to a first wiring, and the other thereof is electrically connected to a second wiring. One of a source and a drain of the second transistor is electrically connected to the first wiring, a gate of the second transistor is electrically connected to a gate of the first transistor, and the other of the source and the drain of the second transistor is electrically connected to one electrode of the capacitor, while the other electrode of the capacitor is electrically connected to a third wiring. The first and second transistors have the same conductivity type.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/417,864, filed May 21, 2019, now allowed, which is a continuation ofU.S. application Ser. No. 15/795,321, filed Oct. 27, 2017, now U.S. Pat.No. 10,304,872, which is a continuation of U.S. application Ser. No.15/247,995, filed Aug. 26, 2016, now U.S. Pat. No. 9,806,107, which is acontinuation of U.S. application Ser. No. 14/594,256, filed Jan. 12,2015, now U.S. Pat. No. 9,432,016, which is a continuation of U.S.application Ser. No. 14/222,822, filed Mar. 24, 2014, now U.S. Pat. No.8,941,416, which is a continuation of U.S. application Ser. No.13/606,440, filed Sep. 7, 2012, now U.S. Pat. No. 8,736,315, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2011-217150 on Sep. 30, 2011, all of which are incorporatedby reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to semiconductor devicesand display devices.

2. Description of the Related Art

The increase in size of display devices such as liquid crystal displaydevices and EL display devices promotes the development of displaydevices with higher added value. In particular, techniques by which adriver circuit in a display device is composed of only transistorshaving the same conductivity type have been actively developed (seePatent Document 1 and Non-Patent Document 1).

FIG. 17A illustrates a driver circuit disclosed in Patent Document 1.The driver circuit disclosed in Patent Document 1 is composed oftransistors M1, M2, M3, and M4. When a signal IN is at high level, thetransistor M1 is turned off and the transistors M2 to M4 are turned on.Thus, a signal OUT exists in high level. On the other hand, when thesignal IN is at low level, the transistor M1 is turned on, thetransistors M2 and M4 are turned off, and the transistor M3 istemporarily turned on and then turned off. Thus, the signal OUT is atlow level.

FIG. 17B illustrates a driver circuit disclosed in Non-PatentDocument 1. The driver circuit disclosed in Non-Patent Document 1 iscomposed of transistors M11 to M19 and a capacitor C11. When a signal INis at high level, the transistors M12, M14, M16, and M17 are turned on;the transistors M11, M13, and M15 are turned off; and the transistorsM18 and M19 are temporarily turned on and then turned off. Thus, asignal OUT becomes low. On the other hand, when the signal IN is at lowlevel, the transistors M12, M14, M16, M17, and M18 are turned off; thetransistors M11, M15, and M19 are turned on; and the transistor M13 istemporarily turned on and then turned off. Thus, the signal OUT is setat high level.

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2002-328643-   Non-Patent Document 1: Eri Fukumoto, Toshiaki Arai, Narihiro    Morosawa, Kazuhiko Tokunaga, Yasuhiro Terai, Takashige Fujimori, and    Tatsuya Sasaoka, “High Mobility Oxide Semiconductor TFT for Circuit    Integration of AM-OLED,” IDW'10, pp. 631-634

SUMMARY OF THE INVENTION

In the driver circuit disclosed in Patent Document 1, both thetransistors M3 and M4 are turned on when the signal IN is at high level.For that reason, a current flows to a wiring supplied with a potentialVSS via the transistors M3 and M4 in this order from a wiring suppliedwith a potential VDD in a period during which the signal IN is at highlevel, whereby power consumption is increased.

In addition, in the driver circuit disclosed in Patent Document 1, thepotential of a gate of the transistor M1 needs to be low enough to turnoff the transistor M1 in a period during which the signal IN is at highlevel. Consequently, the ratio of channel width (W) to channel length(L) (hereinafter referred to as “W/L”) of the transistor M4 needs to besufficiently larger than that of the transistor M3. However, it is notalways easy to increase W/L of the transistor M3 because increase in W/Lof the transistor M3 simultaneously requires increase in W/L of thetransistor M4, leading to magnify the layout area. For that reason, whenthe transistor M3 is turned on and the potential VDD is supplied to thegate of the transistor M1 in a period during which the signal IN is athigh level, it takes a long time for the potential of the gate of thetransistor M1 to reach a predetermined potential. Accordingly, thetiming of turning on the transistor M1 is delayed and Vgs of thetransistor M1 is decreased, so that the rise time of the signal OUT isextended. As a result, delay, distortion, or the like of the signal OUToccurs.

As is clear from comparison with the driver circuit disclosed in PatentDocument 1, the driver circuit disclosed in Non-Patent Document 1requires a large number of elements including transistors andcapacitors.

In view of the above technical background, an object of one embodimentof the present invention is to reduce a current flowing between wiringsof a circuit via a transistor to reduce power consumption thereof.Another object is to shorten the rise time of an output signal from acircuit to reduce delay or distortion of the output signal. Anotherobject is to reduce the number of elements such as transistors andcapacitors in a circuit. Still another object is to provide a novelcircuit configuration. Note that an object and an effect areinseparable, and it is apparent that an effect described in thisspecification and the like is accompanied by an object associated withthe effect. On the other hand, it is apparent that an object describedin this specification and the like is accompanied by an effectassociated with the object.

According to one embodiment of the present invention, a semiconductordevice includes: a first transistor having a source and a drain one ofwhich is electrically connected to a first wiring and the other of whichis electrically connected to a second wiring; a second transistor havinga source and a drain one of which is electrically connected to the firstwiring, and a gate electrically connected to a gate of the firsttransistor; and a capacitor having a pair of electrodes one of which iselectrically connected to a third wiring and the other of which iselectrically connected to the other of the source and the drain of thesecond transistor.

In the embodiment of the present invention, W/L (W: channel width, L:channel length) of the first transistor may be higher than that of thesecond transistor.

In the embodiment of the present invention, the first transistor and thesecond transistor may have the same conductivity type.

According to one embodiment of the present invention, a current flowingbetween wirings of a circuit via a transistor can be reduced, whichresults in reduction in power consumption thereof. In addition, the risetime of an output signal from a circuit can be shortened, so that delayor distortion of the output signal can be reduced. Moreover, the numberof elements such as transistors and capacitors can be reduced in acircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams for explaining an inverter circuitaccording to one embodiment of the present invention;

FIGS. 2A and 2B each illustrate an inverter circuit according to oneembodiment of the present invention;

FIGS. 3A and 3B each illustrate an inverter circuit according to oneembodiment of the present invention;

FIGS. 4A to 4F are diagrams each illustrating a circuit used in aninverter circuit according to one embodiment of the present invention;

FIGS. 5A and 5B each illustrate an inverter circuit according to oneembodiment of the present invention;

FIGS. 6A and 6B each illustrate an inverter circuit according to oneembodiment of the present invention;

FIGS. 7A and 7B are diagrams for explaining a shift register circuitaccording to one embodiment of the present invention;

FIGS. 8A and 8B each illustrate a shift register circuit according toone embodiment of the present invention;

FIGS. 9A and 9B each illustrate a shift register circuit according toone embodiment of the present invention;

FIGS. 10A and 10B each illustrate a shift register circuit according toone embodiment of the present invention;

FIG. 11 illustrates a shift register circuit according to one embodimentof the present invention;

FIG. 12 illustrates a display device according to one embodiment of thepresent invention;

FIGS. 13A to 13D each illustrate a transistor according to oneembodiment of the present invention;

FIG. 14 illustrates a display device according to one embodiment of thepresent invention;

FIGS. 15A to 15E each illustrate an electronic device according to oneembodiment of the present invention;

FIGS. 16A to 16C are diagrams for illustrating a semiconductor deviceaccording to one embodiment of the present invention; and

FIGS. 17A and 17B each illustrate a conventional driver circuit.

DETAILED DESCRIPTION OF THE INVENTION

Examples of embodiments of the present invention will be described belowwith reference to the accompanying drawings. Note that it will bereadily appreciated by those skilled in the art that details of theembodiments can be modified in various ways without departing from thespirit and scope of the present invention. The present invention istherefore not limited to the following description of the embodiments.

Embodiment 1

In Embodiment 1, an inverter circuit (also referred to as “semiconductordevice” or “driver circuit”) according to one embodiment of the presentinvention will be described.

The configuration of an inverter circuit in this embodiment will bedescribed with reference to FIG. 1A.

The inverter circuit illustrated in FIG. 1A includes a circuit 100 and acircuit 200. The circuit 100 is connected to a wiring 11, a wiring 12, awiring 13, a wiring 14, and the circuit 200. The circuit 200 isconnected to the wiring 11, the wiring 13, the wiring 14, and thecircuit 100.

The circuit 100 includes a transistor 101 and a transistor 102. A firstterminal (also referred to as “one of a source and a drain”) of thetransistor 101 is connected to the wiring 11. A second terminal (alsoreferred to as “the other of the source and the drain”) of thetransistor 101 is connected to the wiring 12. A first terminal of thetransistor 102 is connected to the wiring 13. A second terminal of thetransistor 102 is connected to the wiring 12. A gate of the transistor102 is connected to the wiring 14.

The circuit 200 includes a transistor 201, a transistor 202, atransistor 203, and a capacitor 204. A first terminal of the transistor201 is connected to the wiring 11. A gate of the transistor 201 isconnected to a gate of the transistor 101. A first terminal of thetransistor 202 is connected to the wiring 13. A second terminal of thetransistor 202 is connected to a second terminal of the transistor 201.A gate of the transistor 202 is connected to the wiring 14. A firstterminal of the transistor 203 is connected to the wiring 13. A secondterminal of the transistor 203 is connected to the gate of thetransistor 201. A gate of the transistor 203 is connected to the wiring14. A first electrode (also referred to as “one electrode”) of thecapacitor 204 is connected to the wiring 14. A second electrode (alsoreferred to as “the other electrode”) of the capacitor 204 is connectedto the second terminal of the transistor 201.

Note that a node N1 denotes a point where the gate of the transistor101, the gate of the transistor 201, and the second terminal of thetransistor 203 are connected to each other. A node N2 denotes a pointwhere the second terminal of the transistor 201, the second terminal ofthe transistor 202, and the second electrode of the capacitor 204 areconnected to each other.

Note that the transistors included in the inverter circuit in thisembodiment preferably have the same conductivity type. For example, inthe inverter circuit illustrated in FIG. 1A, the transistors 101, 102,and 201 to 203 preferably have the same conductivity type. In thisembodiment, the case where the transistors 101, 102, and 201 to 203 aren-channel transistors is described.

Note that in this specification and the like, the term “connection”means electrical connection and corresponds to a state in which current,voltage, a potential, a signal, charge, or the like can be supplied ortransmitted. The state of being “connected” therefore means not only astate of direct connection but also a state of indirect connectionthrough an element such as a wiring, a conductive film, a resistor, adiode, a transistor, or a switching element, for example.

The wiring 11 (also referred to as “power supply line”) is supplied witha potential VDD and has a function of transmitting the potential VDD.The potential VDD is a constant potential.

The wiring 13 (also referred to as “power supply line”) is supplied witha potential VSS and has a function of transmitting the potential VSS.The potential VSS is a constant potential and lower than the potentialVDD.

The wiring 14 (also referred to as “signal line”) is supplied with asignal IN and has a function of transmitting the signal IN. The signalIN is an input signal of the inverter circuit illustrated in FIG. 1A.The signal IN is a signal for controlling the on/off state of thetransistor 102, the transistor 202, and the transistor 203.

The wiring 12 (also referred to as “signal line”) outputs a signal OUTand has a function of transmitting the signal OUT. The signal OUT is anoutput signal of the inverter circuit illustrated in FIG. 1A.

Without limitation to the above signals and potentials, various othersignals and potentials can be input to the wirings 11, 13, and 14.

The circuit 100 (also referred to as “buffer circuit”) has a function ofsupplying the potential VDD of the wiring 11 to the wiring 12 inaccordance with an output signal of the circuit 200, and a function ofsupplying the potential VSS of the wiring 13 to the wiring 12 inaccordance with the signal IN. That is, the circuit 100 has a functionof supplying one of the potential VDD of the wiring 11 and the potentialVSS of the wiring 13 to the wiring 12 in accordance with the outputsignal of the circuit 200 and the signal IN.

The circuit 200 (also referred to as “control circuit”) has a functionof generating a signal (the potential of the node N1) for controllingthe time at which the circuit 100 supplies the potential VDD of thewiring 11 to the wiring 12, in accordance with the signal IN.

The transistor 101 has a function of controlling electrical continuitybetween the wiring 11 and the wiring 12, a function of supplying thepotential VDD of the wiring 11 to the wiring 12, and a function ofholding a potential difference between the wiring 12 and the node N1.

The transistor 102 has a function of controlling electrical continuitybetween the wiring 13 and the wiring 12, and a function of supplying thepotential VSS of the wiring 13 to the wiring 12.

The transistor 201 has a function of controlling electrical continuitybetween the wiring 11 and the node N2, a function of supplying thepotential VDD of the wiring 11 to the node N2, and a function of holdinga potential difference between the node N1 and the node N2.

The transistor 202 has a function of controlling electrical continuitybetween the wiring 13 and the node N2, and a function of supplying thepotential VSS of the wiring 13 to the node N2.

The transistor 203 has a function of controlling electrical continuitybetween the wiring 13 and the node N1, and a function of supplying thepotential VSS of the wiring 13 to the node N1.

The capacitor 204 has a function of holding a potential differencebetween the wiring 14 and the node N2.

Next, an example of a method of driving the inverter circuit illustratedin FIG. 1A will be described with reference to FIG. 1B. FIG. 1B is anexample of a timing chart illustrating the method of driving theinverter circuit in FIG. 1A.

The following description is made assuming that the signal IN is adigital signal with a high-level potential equal to the potential VDDand a low-level potential equal to the potential VSS. The case where thesignal IN is at high level and the case where the signal IN is at lowlevel are separately described.

First, when the signal IN is set at high level, the transistors 102,202, and 203 are turned on.

When the transistor 203 is turned on, the potential VSS of the wiring 13is supplied to the node N1, so that the potential of the node N1decreases to the potential VSS. When the potential of the node N1decreases to the potential VSS, the transistors 101 and 201 are turnedoff.

When the transistor 202 is turned on, the potential VSS of the wiring 13is supplied to the node N2. Thus, the potential of the node N2 decreasesto the potential VSS.

When the transistor 102 is turned on, the potential VSS of the wiring 13is supplied to the wiring 12. Thus, the potential of the wiring 12decreases to the potential VSS. That is, the signal OUT is changed tolow level.

Then, when the signal IN is set at low level, the transistors 102, 202,and 203 are turned off.

When the transistor 203 is turned off, the node N1 is brought into afloating state. Consequently, the potential of the node N1 remains atthe potential VSS, so that the transistors 101 and 201 are kept off.

When the transistor 202 is turned off, the node N2 is brought into afloating state. At this time, the potential difference between thewiring 14 and the node N2 in the period during which the signal IN is athigh level is held in the capacitor 204. Thus, the potential of the nodeN2 decreases when the signal IN is set at low level. The transistor 201is turned on when the potential of the node N2 decreases to less than apotential obtained by subtracting the threshold voltage of thetransistor 201 from the potential of the node N1 (e.g., the potentialVSS).

When the transistor 201 is turned on, the potential VDD of the wiring 11is supplied to the node N2, so that the potential of the node N2 rises.At this time, the potential difference between the node N1 and the nodeN2 at the time when the transistor 202 is off is held between the gateand the second terminal of the transistor 201. Consequently, thepotential of the node N1 also rises along with the increase in thepotential of the node N2. The potential of the node N2 rises to thepotential VDD, and the potential of the node N1 exceeds the potentialVDD. This is called bootstrap. Then, the transistor 101 is turned on bythe increase in the potential of the node N1.

When the transistor 101 is turned on, the potential VDD of the wiring 11is supplied to the wiring 12. Moreover, the potential of the node N1exceeds the potential VDD as has been described. Consequently, thepotential of the wiring 12 increases to the potential VDD. That is, thesignal OUT becomes high.

As described above, the inverter circuit in FIG. 1A does not have aperiod during which both the transistors 101 and 102 are on or a periodduring which both the transistors 201 and 202 are on, therebyeliminating a path through which current flows between the wirings 11and 13. Further, the high-level potential of the signal OUT can beincreased to the potential VDD of the wiring 11 with a smaller number oftransistors than a conventional driver circuit.

When the signal IN is set at low level, the potential of the node N1rises along with the increase in the potential of the second terminal ofthe transistor 101 as well as the increase in the potential of thesecond terminal of the transistor 201. As a result, the time requiredfor the potential of the node N1 to reach a predetermined potential canbe shortened, so that the timing of turning on the transistor 101 can beadvanced. Moreover, since the potential of the node N1 can be madehigher, Vgs of the transistor 101 can be further increased. The risetime of the signal OUT can be significantly shortened with a synergisticinteraction of the ability of the inverter circuit in FIG. 1A to advancetiming for turning on the transistor 101 and the ability to increase Vgsof the transistor 101.

Next, inverter circuits different from the one in FIG. 1A will bedescribed with reference to FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 4Ato 4F, FIGS. 5A and 5B, and FIGS. 6A and 6B.

The inverter circuit illustrated in FIG. 2A has a configuration in whicha circuit 300A is provided in the inverter circuit in FIG. 1A.

A first terminal (also referred to as “input terminal”) of the circuit300A is connected to the wiring 14. A second terminal (also referred toas “output terminal”) of the circuit 300A is connected to the gate ofthe transistor 203.

The circuit 300A has a function of outputting, from the second terminal,a signal corresponding to a signal input to the first terminal (e.g.,the signal IN) and a function of outputting, from the second terminal, asignal that is delayed and/or distorted compared to the signal input tothe first terminal.

Note that for example, the expression “a second signal is delayedcompared to a first signal” means that the timing of rising or fallingof the second signal is later than that of the first signal. Further,for example, the expression “the second signal is distorted compared tothe first signal” means that the rise time or fall time of the secondsignal is longer than that of the first signal.

In the inverter circuit in FIG. 2A, the signal output from the secondterminal of the circuit 300A remains at high level for a predeterminedperiod after the signal IN changes from high level to low level. Inother words, the transistor 203 is kept on and the potential VSScontinues to be supplied to the node N1 for a predetermined period afterthe signal IN changes from high level to low level.

Accordingly, in the inverter circuit in FIG. 2A, the potential VSS ofthe wiring 13 can be supplied to the node N1 when the potential of thenode N2 is decreased by capacitive coupling caused by the capacitor 204,thereby suppressing the decrease in the potential of the node N1 alongwith the decrease in the potential of the node N2. That is, thepotential difference between the node N1 and the node N2 can beincreased so that the potential of the node N1 at the time when thepotential of the node N2 becomes the potential VDD can be made higherand Vgs of the transistor 101 can be further increased. Consequently,the rise time of the signal OUT can be shortened.

Note that in the inverter circuit in FIG. 2A, the first electrode of thecapacitor 204 may be connected to the second terminal of the circuit300A.

The inverter circuit illustrated in FIG. 2B has a configuration in whicha circuit 300B is provided in the inverter circuit in FIG. 2A.

A first terminal of the circuit 300B is connected to the wiring 14. Asecond terminal of the circuit 300B is connected to the first electrodeof the capacitor 204.

The circuit 300B has functions similar to those of the circuit 300A.However, it is preferable that a signal output from the second terminalof the circuit 300B be not delayed and/or distorted largely compared toa signal output from the second terminal of the circuit 300A.

In the inverter circuit in FIG. 2B, the signals output from the secondterminal of the circuit 300A and the second terminal of the circuit 300Bremain at high level for a predetermined period after the signal INchanges from high level to low level. In other words, the transistor 203is kept on and the potential VSS continues to be supplied to the node N1for a predetermined period after the signal IN changes from high levelto low level. In addition, a signal input to the first electrode of thecapacitor 204 remains at high level for a predetermined period.

After that, even after the signal output from the second terminal of thecircuit 300B changes from high level to low level, the signal outputfrom the circuit 300A remains at high level for a predetermined period.In other words, the transistor 203 is kept on and the potential VSScontinues to be supplied to the node N1 for a predetermined period afterthe signal output from the second terminal of the circuit 300B changesfrom high level to low level.

Consequently, in the inverter circuit in FIG. 2B, the potential of thefirst electrode of the capacitor 204 can be lowered after the transistor202 is turned off. In other words, the potential of the node N2 can belowered by capacitive coupling resulting from the capacitor 204 afterthe node N2 is assuredly brought into a floating state. Thus, thepotential of the node N2 can be further lowered. Further, as in theinverter circuit in FIG. 2A, the potential VSS of the wiring 13 can besupplied to the node N1 in the inverter circuit in FIG. 2B when thepotential of the node N2 is lowered by capacitive coupling caused by thecapacitor 204, thereby suppressing the decrease in the potential of thenode N1 along with the decrease in the potential of the node N2.

The potential difference between the node N1 and the node N2 can befurther increased with a synergistic interaction of the ability of theinverter circuit in FIG. 2B to further lower the potential of the nodeN2 and the ability to suppress the decrease in the potential of the nodeN1. A larger potential difference between the node N1 and the node N2can further increase the potential of the node N1 at the time when thepotential of the node N2 becomes the potential VDD, resulting in furtherincrease in Vgs of the transistor 101. Consequently, the rise time ofthe signal OUT can be further shortened.

The inverter circuit illustrated in FIG. 3A has a configuration in whicha circuit 300C is provided in the inverter circuit in FIG. 2A.

A first terminal of the circuit 300C is connected to the wiring 14. Asecond terminal of the circuit 300C is connected to the first terminalof the circuit 300A and the first electrode of the capacitor 204.

The circuit 300C has functions similar to those of the circuit 300A.

In the inverter circuit in FIG. 3A, signals output from the secondterminal of the circuit 300A and the second terminal of the circuit 300Cremain at high level for a predetermined period after the signal INchanges from high level to low level. In other words, the transistor 203is kept on and the potential VSS continues to be supplied to the node N1for a predetermined period after the signal IN changes from high levelto low level. In addition, a signal input to the first electrode of thecapacitor 204 remains at high level for a predetermined period.

After that, even after the signal output from the second terminal of thecircuit 300C changes from high level to low level, the signal outputfrom the circuit 300A remains at high level for a predetermined period.In other words, the transistor 203 is kept on and the potential VSScontinues to be supplied to the node N1 for a predetermined period afterthe signal output from the second terminal of the circuit 300C changesfrom high level to low level.

Thus, the inverter circuit in FIG. 3A can operate in a manner similar tothat of the inverter circuit in FIG. 2B, and therefore can obtainadvantageous effects similar to those of the inverter circuit in FIG.2B.

Since the circuits 300A and 300C are connected in series in the invertercircuit illustrated in FIG. 3A, the signal output from the secondterminal of the circuit 300A is delayed and/or distorted largelycompared to the signal output from the second terminal of the circuit300C. Consequently, the size of the circuit 300A or the size of elementsincluded in the circuit 300A can be reduced.

The inverter circuit illustrated in FIG. 3B has a configuration in whichthe gate of the transistor 102 is connected to the gate of thetransistor 203 of the inverter circuit in FIG. 2A.

In the inverter circuit in FIG. 3B, the timing of turning on thetransistor 102 can be delayed compared to the case where the gate of thetransistor 102 is connected to the wiring 14 without the circuit 300A.As a result, the time during which both the transistors 101 and 102 areon can be shortened. In other words, the through current flowing betweenthe wirings 11 and 13 can be suppressed. Thus, power consumption can bereduced.

Note that as in the inverter circuit in FIG. 3B, the gate of thetransistor 102 may be connected to the gate of the transistor 203 in theinverter circuits illustrated in FIGS. 2B and 3A.

Specific examples of configurations of the circuits 300A to 300C will bedescribed with reference to FIGS. 4A to 4F. FIGS. 4A to 4F eachillustrate a circuit 300 that can be used as the circuits 300A to 300C.

The circuit 300 illustrated in FIG. 4A includes a resistor 301.

One terminal of the resistor 301 is connected to a first terminal of thecircuit 300, and the other terminal of the resistor 301 is connected toa second terminal of the circuit 300.

The circuit 300 illustrated in FIG. 4B has a configuration in which acapacitor 302 is provided in the circuit 300 in FIG. 4A.

A first electrode of the capacitor 302 is connected to the wiring 13,and a second electrode of the capacitor 302 is connected to the secondterminal of the circuit 300.

Note that the first electrode of the capacitor 302 may be connected tothe wiring 11, the wiring 14, or the like.

Note that the second electrode of the capacitor 302 may be connected tothe first terminal of the circuit 300.

The circuit 300 illustrated in FIG. 4C includes a transistor 303.

A first terminal of the transistor 303 is connected to the firstterminal of the circuit 300. A second terminal of the transistor 303 isconnected to the second terminal of the circuit 300. A gate of thetransistor 303 is connected to the wiring 11.

The circuit 300 illustrated in FIG. 4D has a configuration in which atransistor 304 is provided in the circuit 300 in FIG. 4C.

A first terminal of the transistor 304 is connected to the firstterminal of the circuit 300. A second terminal of the transistor 304 isconnected to the second terminal of the circuit 300. A gate of thetransistor 304 is connected to the first terminal of the circuit 300.

In the circuit 300 in FIG. 4D, the transistor 303 is turned on and thetransistor 304 is turned off when a signal input to the first terminalis at low level. On the other hand, when the signal input to the firstterminal is at high level, both the transistors 303 and 304 are turnedon.

Thus, when the signal input to the first terminal is at low level, thecircuit 300 in FIG. 4D can delay the inputted signal and output theresulting signal from the second terminal. On the other hand, when thesignal input to the first terminal is at high level, the circuit 300 inFIG. 4D can output the signal from the second terminal with negligiblesignal delay.

Note that the transistor 304 may be provided in the circuit 300illustrated in FIGS. 4A and 4B.

The circuit 300 illustrated in FIG. 4E has a configuration in which atransistor 305 is provided in the circuit 300 in FIG. 4C.

A first terminal of the transistor 305 is connected to the wiring 11. Asecond terminal of the transistor 305 is connected to the secondterminal of the circuit 300. A gate of the transistor 305 is connectedto the first terminal of the circuit 300.

In the circuit 300 in FIG. 4E, the transistor 303 is turned on and thetransistor 305 is turned off when a signal input to the first terminalis at low level. On the other hand, when the signal input to the firstterminal is at high level, both the transistors 303 and 305 are turnedon.

Thus, the circuit 300 in FIG. 4E can have advantageous effects similarto those of the circuit 300 in FIG. 4D.

Note that the transistor 305 may be provided in the circuit 300illustrated in FIGS. 4A, 4B, and the like.

The circuit 300 illustrated in FIG. 4F has a configuration in which atransistor 306 and a transistor 307 are provided in the circuit 300 inFIG. 4C.

A first terminal of the transistor 306 is connected to the wiring 11. Asecond terminal of the transistor 306 is connected to the secondterminal of the circuit 300. A first terminal of the transistor 307 isconnected to the first terminal of the circuit 300. A second terminal ofthe transistor 307 is connected to a gate of the transistor 306. A gateof the transistor 307 is connected to the wiring 11.

In the circuit 300 in FIG. 4F, the transistor 303 is turned on and thetransistor 306 is turned off when a signal input to the first terminalis at low level. On the other hand, when the signal input to the firstterminal is at high level, both the transistors 303 and 306 are turnedon. Note that when the signal input to the first terminal is at highlevel, the potential of the gate of the transistor 306 is made higherthan the potential VDD by bootstrap operation.

Consequently, in the circuit 300 in FIG. 4F, which has advantageouseffects similar to those of the circuit 300 in FIG. 4D, a high-levelpotential of a signal output from the second terminal can be thepotential VDD. Further, in the circuit 300 in FIG. 4F, signal delaycaused when the signal input to the first terminal is at high level canbe smaller than that in the circuit 300 in FIG. 4D.

When the circuit 300 in FIG. 4F is used in the inverter circuit in FIG.2A, the first electrode of the capacitor 204 may be connected to thegate of the transistor 306. Since the difference between the highestpotential and the lowest potential of the gate of the transistor 306 islarger than the amplitude voltage of the signal IN, the potential of thenode N2 can be further lowered.

Note that the transistors 306 and 307 may be provided in the circuit 300illustrated in FIGS. 4A, 4B, and the like.

It is preferable that the conductivity type of the transistors includedin the circuit 300 (e.g., the transistors 304 to 307) be the same asthat of the transistor 101.

Note that the circuits 300A to 300C do not necessarily have the sameconfiguration, and each of them can have any of the configurationsillustrated in FIGS. 4A to 4F as appropriate.

FIG. 5A illustrates an example of the configuration of the invertercircuit in which the circuit 300 illustrated in FIG. 4D is used as thecircuit 300A in the inverter circuit of FIG. 2A.

FIG. 5B illustrates an example of the configuration of the invertercircuit in which the circuit 300 illustrated in FIG. 4F is used as thecircuit 300A in the inverter circuit of FIG. 2A.

The inverter circuit illustrated in FIG. 6A has a configuration in whicha transistor 205 is provided in the inverter circuit in FIG. 1A.

A first terminal of the transistor 205 is connected to the secondterminal of the transistor 203. A second terminal of the transistor 205is connected to the gate of the transistor 101 and the gate of thetransistor 201. A gate of the transistor 205 is connected to the wiring11.

The transistor 205 has a function of controlling electrical continuitybetween the second terminal of the transistor 203 and the gates of thetransistors 101 and 201.

In the inverter circuit illustrated in FIG. 6A, in a period during whichthe signal IN is at low level, the transistor 205 is turned off when thepotential of the second terminal of the transistor 203 increases to apotential obtained by subtracting the threshold voltage of thetransistor 205 from the potential of the gate of the transistor 205 (thepotential VDD). Thus, the potential of the second terminal of thetransistor 203 can be lowered, so that deterioration and/or breakdown ofthe transistor 203 can be prevented.

As in the inverter circuit in FIG. 6A, the transistor 205 may beprovided in the inverter circuits illustrated in FIGS. 2A and 2B, FIGS.3A and 3B, and FIGS. 5A and 5B.

The inverter circuit illustrated in FIG. 6B has a configuration in whicheach of the wirings 11 and 13 in the inverter circuit in FIG. 1A isdivided into a plurality of wirings.

The wiring 11 is divided into a wiring 11A and a wiring 11B. The firstterminal of the transistor 101 is connected to the wiring 11A. The firstterminal of the transistor 201 is connected to the wiring 11B. Thewiring 13 is divided into a wiring 13A, a wiring 13B, and a wiring 13C.The first terminal of the transistor 102 is connected to the wiring 13A.The first terminal of the transistor 202 is connected to the wiring 13B.The first terminal of the transistor 203 is connected to the wiring 13C.

The inverter circuit illustrated in FIG. 6B can operate in a mannersimilar to that in FIG. 1A when the potential VDD is supplied to thewirings 11A and 11B and the potential VSS is supplied to the wirings 13Ato 13C. Note that different potentials may be supplied to the wirings11A and 11B and that different potentials may be supplied to the wirings13A to 13C.

Note that only one of the wirings 11 and 13 may be divided into aplurality of wirings.

When the wiring 13 is divided into a plurality of wirings, it ispossible that the wiring 13C is omitted and the first terminal of thetransistor 203 is connected to the wiring 13A or the wiring 13B.Alternatively, it is possible that the wiring 13A is omitted and thefirst terminal of the transistor 102 is connected to the wiring 13B orthe wiring 13C.

As in the inverter circuit in FIG. 6B, the wiring 11 and/or the wiring13 may be divided into a plurality of wirings in the inverter circuitsillustrated in FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 5A and 5B, andFIG. 6A.

Although not illustrated, the inverter circuit illustrated in any ofFIG. 1A, FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 5A and 5B, and FIGS. 6Aand 6B may include a capacitor having a first electrode connected to thesecond terminal of the transistor 101 and a second electrode connectedto the gate of the transistor 101.

Although not illustrated, the inverter circuit illustrated in any ofFIG. 1A, FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 5A and 5B, and FIGS. 6Aand 6B may include a capacitor having a first electrode connected to thesecond terminal of the transistor 201 and a second electrode connectedto the gate of the transistor 201.

Note that a load driven by the transistor 101 (e.g., a load connected tothe wiring 12) is larger than a load driven by the transistors 201 to203 (e.g., a load connected to the node N1 or the node N2). The risetime of the signal OUT can be shortened as W/L of the transistor 101increases. Thus, W/L of the transistor 101 is preferably higher thanthat of the transistors 201 to 203.

Similarly, a load driven by the transistor 102 (e.g., a load connectedto the wiring 12) is larger than a load driven by the transistors 201 to203. The fall time of the signal OUT can be shortened as W/L of thetransistor 102 increases. Thus, W/L of the transistor 102 is preferablyhigher than that of the transistors 201 to 203.

Note also that Vgs of the transistor 101 at which the transistor 101 isturned on is often lower than Vgs of the transistor 102 at which thetransistor 102 is turned on. Therefore, W/L of the transistor 101 ispreferably higher than that of the transistor 102. That is, thetransistor 101 preferably has the highest W/L among the transistorsincluded in the inverter circuit of this embodiment.

The inverter circuit in this embodiment operates normally when thelow-level potential of the signal IN is low enough to turn off thetransistors 102, 202, and 203. For that reason, the low-level potentialof the signal IN may be lower than the potential VSS, in which case Vgsof the transistors 201 to 203 at which the transistors 201 to 203 areturned off can be negative voltage. As a result, the inverter circuitcan operate normally even if the transistors 201 to 203 are normally-ontransistors or if the drain current of the transistors 201 to 203 at thetime when the potential difference between their gates and sources is 0[V] is high.

The inverter circuit in this embodiment operates normally when thehigh-level potential of the signal IN is high enough to turn on thetransistors 102, 202, and 203. For that reason, the high-level potentialof the signal IN may be lower than the potential VDD, in which case thevoltage for driving a circuit that outputs signals to the wiring 14 canbe lowered. In addition, in the inverter circuit of this embodiment, thehigh-level potential of the signal OUT can be the potential VDD even ifthe high-level potential of the signal IN is lower than the potentialVDD.

The signal IN is not limited to a digital signal as long as it has apotential for turning off the transistors 102, 202, and 203 and apotential for turning on the transistors 102, 202, and 203. For example,the signal IN may have three or more potentials or may be an analogsignal.

When a signal such as a clock signal is input to the wiring 11, thesignal of the wiring 11 can be output to the wiring 12 in the case wherethe signal IN is at low level. Specifically, in the case where thewiring 11 is divided into the wirings 11A and 11B as in the invertercircuit illustrated in FIG. 6B, it is preferable that a signal such as aclock signal be input to the wiring 11A and the potential VDD besupplied to the wiring 11B. Thus, the potential of the node N1 can beset high, so that the transistor 101 is likely to be turned on.Consequently, the signal of the wiring 11A can be output to the wiring12 in a stable manner.

The inverter circuit in this embodiment operates normally when thewiring 13 is supplied with a low-level signal in a period during whichthe transistors 102, 202, and 203 are on (e.g., a period during whichthe signal IN is at high level). When the wiring 13 is supplied with ahigh-level signal in all or part of a period during which thetransistors 102, 202, and 203 are off (e.g., a period during which thesignal IN is at low level), a reverse bias can be applied to thetransistors 102, 202, and 203. Thus, deterioration of the transistors102, 202, and 203 can be suppressed.

Here, a semiconductor device having the following configuration is oneembodiment of the present invention.

One embodiment of the present invention is a semiconductor deviceincluding the transistor 101, the transistor 201, and the capacitor 204.The first terminal of the transistor 101 is connected to the wiring 11.The second terminal of the transistor 101 is connected to the wiring 12.The first terminal of the transistor 201 is connected to the wiring 11.The gate of the transistor 201 is connected to the gate of thetransistor 101. The first electrode of the capacitor 204 is connected tothe wiring 14. The second electrode of the capacitor 204 is connected tothe second terminal of the transistor 201 (see FIG. 16A).

In the above embodiment of the present invention, the potential of thesecond terminal of the transistor 201 falls along with the decrease inthe potential of the wiring 14. By the decrease in the potential of thesecond terminal of the transistor 201, the transistor 201 is turned onand the potential of the wiring 11 is supplied to the second terminal ofthe transistor 201, resulting in the increase in the potential of thesecond terminal of the transistor 201 (see FIG. 16B). Moreover, thepotential of the gate of the transistor 201 rises along with theincrease in the potential of the second terminal of the transistor 201.By the increase in the potential of the gate of the transistor 201, thetransistor 101 is turned on and the potential of the wiring 11 issupplied to the wiring 12, so that the potential of the wiring 12 rises(see FIG. 16C).

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 2

In Embodiment 2, a shift register circuit (also referred to as“semiconductor device” or “driver circuit”) according to one embodimentof the present invention will be described.

A shift register circuit in this embodiment includes a plurality offlip-flop circuits (also referred to as “semiconductor devices” or“driver circuits”). First, a flip-flop circuit will be described, andthen a shift register circuit including the flip-flop circuit will bedescribed.

A flip-flop circuit included in the shift register circuit of thisembodiment will be described with reference to FIG. 7A.

The flip-flop circuit in FIG. 7A includes a transistor 401, a transistor402, a transistor 403, a transistor 404, a transistor 405, and a circuit500. A first terminal of the transistor 401 is connected to a wiring 21.A second terminal of the transistor 401 is connected to a wiring 22. Afirst terminal of the transistor 402 is connected to the wiring 13. Asecond terminal of the transistor 402 is connected to the wiring 22. Afirst terminal of the transistor 403 is connected to the wiring 13. Asecond terminal of the transistor 403 is connected to a gate of thetransistor 401. A first terminal of the transistor 404 is connected to awiring 23. A second terminal of the transistor 404 is connected to thegate of the transistor 401. A gate of the transistor 404 is connected tothe wiring 23. A first terminal of the transistor 405 is connected tothe wiring 13. A second terminal of the transistor 405 is connected tothe gate of the transistor 401. A gate of the transistor 405 isconnected to a wiring 24. A first terminal (also referred to as “inputterminal”) of the circuit 500 is connected to the gate of the transistor401. A second terminal (also referred to as “output terminal”) of thecircuit 500 is connected to a gate of the transistor 402 and a gate ofthe transistor 403.

The circuit 500 can be the inverter circuit described in Embodiment 1.The first terminal of the circuit 500 corresponds to the wiring 14 inthe inverter circuit of Embodiment 1, and the second terminal of thecircuit 500 corresponds to the wiring 12 in the inverter circuit ofEmbodiment 1.

Note that a node N3 denotes a point where the gate of the transistor401, the second terminal of the transistor 403, the second terminal ofthe transistor 404, the second terminal of the transistor 405, and thefirst terminal of the circuit 500 are connected to each other. Inaddition, a node N4 denotes a point where the gate of the transistor402, the gate of the transistor 403, and the second terminal of thecircuit 500 are connected to each other.

Note that the transistors included in the flip-flop circuit in thisembodiment preferably have the same conductivity type. For example, inthe flip-flop circuit illustrated in FIG. 7A, the transistors 401 to 405and the transistors included in the circuit 500 preferably have the sameconductivity type.

The wiring 21 (also referred to as “signal line”) is supplied with asignal CK and has a function of transmitting the signal CK. The signalCK is a clock signal that oscillates between a high and a low state.

The wiring 22 (also referred to as “signal line”) outputs a signal SOUTand has a function of transmitting the signal SOUT. The signal SOUT isan output signal of the flip-flop circuit illustrated in FIG. 7A.

The wiring 23 (also referred to as “signal line”) is supplied with asignal SP and has a function of transmitting the signal SP. The signalSP is an input signal of the flip-flop circuit illustrated in FIG. 7A.

The wiring 24 (also referred to as “signal line”) is supplied with asignal RE and has a function of transmitting the signal RE. The signalRE is an input signal of the flip-flop circuit illustrated in FIG. 7A.

Without limitation to the above signals or potentials, various othersignals and potentials can be input to the wirings 21, 23, and 24.

The transistor 401 has a function of controlling electrical continuitybetween the wiring 21 and the wiring 22, a function of supplying thesignal CK of the wiring 21 to the wiring 22, and a function of holding apotential difference between the wiring 22 and the node N3.

The transistor 402 has a function of controlling electrical continuitybetween the wiring 13 and the wiring 22, and a function of supplying thepotential VS S of the wiring 13 to the wiring 22.

The transistor 403 has a function of controlling electrical continuitybetween the wiring 13 and the node N3, and a function of supplying thepotential VSS of the wiring 13 to the node N3.

The transistor 404 has a function of controlling electrical continuitybetween the wiring 23 and the node N3, and a function of supplying thesignal SP of the wiring 23 to the node N3.

The transistor 405 has a function of controlling electrical continuitybetween the wiring 13 and the node N3, and a function of supplying thepotential VSS to the node N3.

Next, an example of a method of driving the flip-flop circuitillustrated in FIG. 7A will be described with reference to FIG. 7B. FIG.7B is an example of a timing chart illustrating the method of drivingthe flip-flop circuit in FIG. 7A.

The following description is made assuming that the signal CK, thesignal SP, and the signal RE are digital signals each having ahigh-level potential equal to the potential VDD and a low-levelpotential equal to the potential VSS. The operations of the flip-flopcircuit in periods Ta, Tb, Tc, and Td are separately described.

In the period Ta, the signal SP is set at high level, the signal RE isset at low level, and the signal CK is set at low level. Thus, thetransistor 404 is turned on and the transistor 405 is turned off.

When the transistor 404 is turned on, the signal SP of the wiring 23 issupplied to the node N3. Since the signal SP is at high level, thepotential of the node N3 rises. When the potential of the node N3increases, the output signal of the circuit 500 becomes low. Thus, thetransistors 402 and 403 are turned off. Further, the transistor 401 isturned on by the increase in the potential of the node N3.

When the transistor 401 is turned on, the signal CK of the wiring 21 issupplied to the wiring 22. Since the signal CK is at low level, thepotential of the wiring 22 becomes the potential VSS. That is, thesignal SOUT exists in low level.

The transistor 404 is turned off when the potential of the node N3increases to a potential obtained by subtracting the threshold voltageof the transistor 404 from the gate potential of the transistor 404 (thepotential VDD). Thus, the node N3 is brought into a floating state.

Then, in the period Tb, the signal SP is set at low level, the signal REis kept at low level, and the signal CK is set at high level. Thus, thetransistors 404 and 405 are kept off, and the output signal of thecircuit 500 remains at low level. Consequently, the transistors 402 and403 are kept off.

Since the transistors 403 to 405 remain off, the node N3 is kept in afloating state. As a result, the potential of the node N3 is kept high,so that the transistor 401 is kept on.

Since the transistor 401 remains on, the signal CK of the wiring 21continues to be supplied to the wiring 22. The potential of the wiring22 starts to rise because the signal CK is at high level. At this time,the potential difference between the node N3 and the wiring 22 in theperiod Ta is held between the gate and second terminal of the transistor401. Thus, the potential of the node N3 rises along with the increase inthe potential of the wiring 22. As a result, the potential of the wiring22 increases to the potential VDD, which is equal to the high-levelpotential of the signal CK. That is, the signal SOUT becomes high.

Then, in the period Tc, the signal SP remains at low level, the signalRE is set at high level, and the signal CK is set at low level. Thus,the transistor 404 is kept off and the transistor 405 is turned on.

When the transistor 405 is turned on, the potential VSS of the wiring 13is supplied to the node N3. Thus, the potential of the node N3 decreasesto the potential VSS, so that the transistor 401 is turned off.Moreover, the output signal of the circuit 500 becomes high, and thetransistors 402 and 403 are turned on.

When the transistor 402 is turned on, the potential VSS of the wiring 13is supplied to the wiring 22. Thus, the potential of the wiring 22decreases to the potential VSS. That is, the signal SOUT is changed tolow level.

Then, in the period Td, the signal SP remains at low level, the signalRE is set at low level, and the signal CK oscillates between high andlow levels. Thus, the transistor 404 is kept off and the transistor 405is turned off, and the output signal of the circuit 500 remains at highlevel. Consequently, the transistors 402 and 403 are kept on.

The potential VSS of the wiring 13 continues to be supplied to the nodeN3 while the transistor 403 is kept on. Thus, the potential of the nodeN3 remains at the potential VSS, so that the transistor 401 is kept off.

The potential VSS of the wiring 13 continues to be supplied to thewiring 22 while the transistor 402 is kept on. Thus, the potential ofthe wiring 22 remains at the potential VSS. That is, the signal SOUTremains at low level.

As described above, by including the inverter circuit described inEmbodiment 1, the flip-flop circuit illustrated in FIG. 7A can obtainadvantageous effects similar to those of the inverter circuit inEmbodiment 1.

Next, flip-flop circuits different from the one in FIG. 7A will bedescribed with reference to FIGS. 8A and 8B and FIGS. 9A and 9B. Notethat a description of differences from FIG. 7A will be given below.

The flip-flop circuit illustrated in FIG. 8A has a configuration inwhich a transistor 406 is provided in the flip-flop circuit in FIG. 7A.

A first terminal of the transistor 406 is connected to the wiring 13. Asecond terminal of the transistor 406 is connected to the wiring 22. Agate of the transistor 406 is connected to a wiring 25.

The wiring 25 (also referred to as “signal line”) is supplied with asignal CKB and has a function of transmitting the signal CKB. The signalCKB is a signal whose phase is inverted with respect to the signal CK ora signal that is out of phase with the signal CK.

The transistor 406 has a function of controlling electrical continuitybetween the wiring 13 and the wiring 22, and a function of supplying thepotential VS S of the wiring 13 to the wiring 22.

In the flip-flop circuit in FIG. 8A, in the period Td, the transistor406 is turned on every time the signal CKB is set at high level.Accordingly, in the period Td, the potential VSS of the wiring 13 issupplied to the wiring 22 every time the signal CKB is set at highlevel.

Specifically, in the case where the signal CKB is the inversion signalof the signal CK, the signal CKB is set at high level and the transistor406 is turned on in the period Ta and the period Tc. Thus, the potentialVSS of the wiring 13 is supplied to the wiring 22 through both thetransistors 402 and 406 in the period Tc, so that the fall time of thesignal SOUT can be shortened.

In the case where the flip-flop circuit includes the transistor 406, thepotential of the wiring 22 can be kept at the potential VSS in theperiod Td. For that reason, the transistor 402 may be omitted, in whichcase the number of transistors and the layout area can be reduced.

The flip-flop circuit illustrated in FIG. 8B has a configuration inwhich a transistor 407 is provided in the flip-flop circuit in FIG. 7A.

A first terminal of the transistor 407 is connected to the wiring 13. Asecond terminal of the transistor 407 is connected to the wiring 22. Agate of the transistor 407 is connected to the wiring 24.

The transistor 407 has a function of controlling electrical continuitybetween the wiring 13 and the wiring 22, and a function of supplying thepotential VS S of the wiring 13 to the wiring 22.

In the flip-flop circuit illustrated in FIG. 8B, the transistor 407 isoff in the period Ta, the period Tb, and the period Td and is on in theperiod Tc. When the transistor 407 is turned on in the period Tc, thepotential VSS of the wiring 13 is supplied to the wiring 22.

Thus, the potential VSS of the wiring 13 is supplied to the wiring 22through both the transistors 402 and 407 in the period Tc, so that thefall time of the signal SOUT can be shortened.

As in the flip-flop circuit in FIG. 8B, the transistor 407 may beprovided in the flip-flop circuit in FIG. 8A.

The flip-flop circuit illustrated in FIG. 9A has a configuration inwhich a transistor 408 is provided in the flip-flop circuit in FIG. 7A.

A first terminal of the transistor 408 is connected to the wiring 11. Asecond terminal of the transistor 408 is connected to the node N4. Agate of the transistor 408 is connected to the wiring 24.

The transistor 408 has a function of controlling electrical continuitybetween the wiring 11 and the node N4, and a function of supplying thepotential VDD of the wiring 11 to the node N4.

In the flip-flop circuit illustrated in FIG. 9A, the transistor 408 isoff in the period Ta, the period Tb, and the period Td and is on in theperiod Tc. When the transistor 408 is turned on in the period Tc, thepotential VDD of the wiring 11 is supplied to the node N4.

Consequently, the time required for the potential of the node N4 toreach a predetermined value can be shortened, so that the timing ofturning on the transistors 402 and 403 can be advanced. As a result, thetiming of supplying the potential VSS of the wiring 13 to the wiring 22is also advanced, so that the fall time of the signal SOUT can beshortened.

As in the flip-flop circuit in FIG. 9A, the transistor 408 may beprovided in the flip-flop circuits in FIGS. 8A and 8B.

In the case where the flip-flop circuit includes the transistor 408, thetransistors 402 and 403 are on in the period Tc. For that reason, thetransistor 405 may be omitted, in which case the number of transistorsand the layout area can be reduced.

Note that the transistor 408 may be used in the flip-flop circuitillustrated in FIG. 8A and its first terminal may be connected to thewiring 25. Even when the first terminal of the transistor 408 isconnected to the wiring 25, the signal CKB of the wiring 25 is at highlevel in the period Tc to allow the transistor 408 to be turned on;therefore, the flip-flop circuit can operate in the above-describedmanner.

The flip-flop circuit illustrated in FIG. 9B has a configuration inwhich a transistor 409 is provided in the flip-flop circuit in FIG. 7A.

A first terminal of the transistor 409 is connected to the wiring 21. Asecond terminal of the transistor 409 is connected to a wiring 26. Agate of the transistor 409 is connected to the node N3.

In the flip-flop circuit illustrated in FIG. 9B, a signal SOUTa denotesa signal output from the wiring 22 and a signal SOUTb denotes a signaloutput from the wiring 26. The signal SOUTb is also an output signal ofthe flip-flop circuit. The wiring 26 (also referred to as “signal line”)has a function of transmitting the signal SOUTb.

The transistor 409 has functions similar to those of the transistor 401,and for example, has a function of controlling electrical continuitybetween the wiring 21 and the wiring 26.

The flip-flop circuit in FIG. 9B can generate the signal SOUTb, which issimilar to the signal SOUTa. Accordingly, for example, the signal SOUTacan be used to drive a load connected to the wiring 22 and the signalSOUTb can be used to drive a flip-flop circuit in a different stageconnected to the wiring 26.

As in the flip-flop circuit in FIG. 9B, the transistor 409 may beprovided in the flip-flop circuits in FIGS. 8A and 8B and FIG. 9A.

Although not illustrated, in the flip-flop circuit such as the onesillustrated in FIG. 7A, FIGS. 8A and 8B, and FIGS. 9A and 9B, the firstterminal of the transistor 404 may be connected to the wiring 11 or thewiring 25. In that case, the node N3 is supplied with the potential orthe signal of the wiring 11 or the wiring 25 in the period Ta, so thatthe load of a circuit that supplies the signal SP to the wiring 23 canbe decreased.

Although not illustrated, the flip-flop circuit such as the onesillustrated in FIG. 7A, FIGS. 8A and 8B, and FIGS. 9A and 9B may includea capacitor having one electrode connected to the wiring 22 and theother electrode connected to the node N3. Providing the capacitor in theflip-flop circuit can increase the capacitance between the gate andsecond terminal of the transistor 401, so that bootstrap operation canbe easily performed.

Although not shown, the flip-flop circuit such as the ones illustratedin FIG. 7A, FIGS. 8A and 8B, and FIGS. 9A and 9B may include atransistor having a first terminal connected to the wiring 22, a secondterminal connected to the node N3, and a gate connected to the wiring21. Accordingly, the potential VSS of the node N3 can be supplied to thewiring 22 or the potential of the wiring 22 can be supplied to the nodeN3 in a period during which the signal CK is at high level within theperiod Td. Consequently, one of the transistors 402 and 403 may beomitted, in which case the load of the circuit 500 is decreased and as aresult, W/L of the transistors included in the circuit 500 can bereduced.

Although not illustrated, the flip-flop circuit such as the onesillustrated in FIG. 7A, FIGS. 8A and 8B, and FIGS. 9A and 9B may includea transistor having a first terminal connected to the wiring 23, asecond terminal connected to the node N3, and a gate connected to thewiring 25. In that case, the potential of the node N3 can be rapidlyincreased in the period Ta.

Although not shown, in the flip-flop circuit such as the onesillustrated in FIG. 7A, FIGS. 8A and 8B, and FIGS. 9A and 9B, it ispossible that a transistor is additionally provided so that its firstelectrode is connected to the second terminal of the transistor 404, itssecond terminal is connected to the gate of the transistor 401, and itsgate is connected to the wiring 11 or the wiring 25, while the secondterminal of the transistor 404 is not connected to the gate of thetransistor 401. Accordingly, voltage applied to the transistor 404 andthe transistor connected to the second terminal of the transistor 404can be lowered, whereby deterioration, breakdown, or the like of thetransistors can be prevented. Note that the first terminal of thecircuit 500 is connected to the second terminal of the transistor 404 orthe gate of the transistor 401, and the second terminal of thetransistor 405 is connected to the second terminal of the transistor 404or the gate of the transistor 401.

Although not shown, the flip-flop circuit illustrated in FIG. 9B or thelike may include a transistor having a first terminal connected to thewiring 13, a second terminal connected to the wiring 26, and a gateconnected to the node N4, the wiring 24, or the wiring 25. In that case,the potential VSS of the wiring 13 can be supplied to the wiring 26,which makes it easier to maintain the potential of the wiring 26 at thepotential VSS.

Next, specific examples of flip-flop circuits in which the invertercircuit in Embodiment 1 is used as the circuit 500 will be described.

A flip-flop circuit illustrated in FIG. 10A has a configuration in whichthe inverter circuit in FIG. 1A is used as the circuit 500 of theflip-flop circuit in FIG. 7A.

A flip-flop circuit illustrated in FIG. 10B has a configuration in whichthe first terminals of the transistors 101 and 201 are connected to thewiring 21 in the flip-flop circuit in FIG. 10A.

In the flip-flop circuit illustrated in FIG. 10B, the potential VSS ofthe wiring 13 is supplied to the node N4 in the period Ta and the periodTb, and the signal CK of the wiring 21 is supplied to the node N4 in theperiod Tc and the period Td. In the period Td, the supply of the signalCK of the wiring 21 to the node N4 makes the potential of the node N4switch repeatedly between the potential VDD and the potential VSS,whereby the transistors 402 and 403 are repeatedly turned on and off. Inother words, in the period Td, the potential VSS of the wiring 13 issupplied to the wiring 22 at fixed intervals, and the time during whichthe transistors 402 and 403 are on is shortened. Thus, the potential ofthe wiring 22 can be maintained at the potential VSS, and deteriorationof the transistors 402 and 403 can be suppressed.

In the case where any of the inverter circuits described in Embodiment 1is used as the circuit 500 in the flip flop circuits such as the onesillustrated in FIGS. 8A and 8B and FIGS. 9A and 9B, the first terminalsof the transistors 101 and 201 may be connected to the wiring 21 as inthe flip-flop circuit in FIG. 10B.

Next, the shift register circuit in this embodiment will be describedwith reference to FIG. 11.

The shift register circuit illustrated in FIG. 11 includes N flip-flopcircuits 600 (N is a natural number). Note that FIG. 11 only illustratesthe flip-flop circuits 600 in first to third stages (flip-flop circuits600_1, 600_2, and 600_3).

In the shift register circuit in FIG. 11, the flip-flop circuitillustrated in FIG. 7A is used as the flip-flop circuit 600; however,the flip-flop circuit 600 is not limited to the flip-flop circuit inFIG. 7A.

The shift register circuit in FIG. 11 is connected to N wirings 31, awiring 32, a wiring 33, and a wiring 34. The i-th stage flip-flopcircuit 600 (i is one of 2 to N−1) is connected to the wirings 31 in thei-th stage, the (i−1)th stage, and the (i+1)th stage and one of thewirings 33 and 34. Further, in the i-th stage flip-flop circuit 600, thewiring 22 is connected to the i-th stage wiring 31; the wiring 23 isconnected to the (i−1)th stage wiring 31; the wiring 24 is connected tothe (i+1)th stage wiring 31; and the wiring 21 is connected to thewiring 33 or the wiring 34.

In the case where the wiring 21 is connected to the wiring 33 in thei-th stage flip-flop circuit 600, the wiring 21 is connected to thewiring 34 in the (i−1)th stage and (i+1)th stage flip-flop circuits 600.

The connection relation in the first stage flip-flop circuit 600 is thesame as that of the i-th stage flip-flop circuit 600, except that thewiring 23 is connected to the wiring 32 in the first stage flip-flopcircuit 600 because there is no (i−1)th stage wiring 31 corresponding tothe first stage flip-flop circuit 600.

The connection relation in the N-th stage flip-flop circuit 600 is thesame as that of the i-th stage flip-flop circuit 600, except that thewiring 24 is connected to the wiring 32 in the N-th stage flip-flopcircuit 600 because there is no (i+1)th stage wiring 31 for the N-thstage flip-flop circuit 600. Note that in the N-th stage flip-flopcircuit 600, the wiring 24 may be connected to the wiring 33, the wiring34, or a wiring to which a signal corresponding to the signal RE isinput.

Signals SOUT_1 to SOUT_N are output from the respective N wirings 31(also referred to as “signal lines”). The N wirings 31 have a functionof transmitting the signals SOUT_1 to SOUT_N. For example, the signalSOUT_i is output from the i-th stage wiring 31, which has a function oftransmitting the signal SOUT_i.

The wiring 32 (also referred to as “signal line”) is supplied with asignal SSP and has a function of transmitting the signal SSP. The signalSSP is a start pulse of the shift register circuit in FIG. 11.

The wiring 33 (also referred to as “signal line”) is supplied with thesignal CK and has a function of transmitting the signal CK.

The wiring 34 (also referred to as “signal line”) is supplied with thesignal CKB and has a function of transmitting the signal CKB.

Without limitation to the above signals or potentials, various othersignals and potentials can be input to the wirings 32 to 34.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 3

Using an EL display device as an example, cross-sectional structures ofa pixel and a driver circuit of a display device according to oneembodiment of the present invention will be described with reference toFIG. 12. FIG. 12 exemplifies cross-sectional structures of a pixel 840and a driver circuit 841.

The pixel 840 includes a light-emitting element 832 and a transistor 831having a function of supplying current to the light-emitting element832. In addition to the light-emitting element 832 and the transistor831, the pixel 840 may also include a variety of semiconductor elementssuch as a transistor that controls input of an image signal to the pixel840 and a capacitor that holds the potential of an image signal.

The driver circuit 841 includes a transistor 830 and a capacitor 833 forholding the gate voltage of the transistor 830. The driver circuit 841corresponds to the inverter circuit in Embodiment 1 or the flip-flopcircuit or the shift register circuit in Embodiment 2, for exampleSpecifically, the transistor 830 corresponds to the transistor 101 inEmbodiment 1 or the transistor 401 in Embodiment 2, for example. Thedriver circuit 841 may also include a variety of semiconductor elementssuch as a transistor and a capacitor in addition to the transistor 830and the capacitor 833.

The transistor 831 includes, over a substrate 800 having an insulatingsurface, a conductive film 816 functioning as a gate, a gate insulatingfilm 802 over the conductive film 816, a semiconductor film 817 thatoverlaps the conductive film 816 with the gate insulating film 802placed therebetween, and conductive films 815 and 818 that arepositioned over the semiconductor film 817 and function as a sourceterminal and a drain terminal. The conductive film 816 also functions asa scan line.

The transistor 830 includes, over the substrate 800 having an insulatingsurface, a conductive film 812 functioning as a gate, the gateinsulating film 802 over the conductive film 812, a semiconductor film813 that overlaps the conductive film 812 with the gate insulating film802 placed therebetween, and conductive films 814 and 819 that arepositioned over the semiconductor film 813 and function as a sourceterminal and a drain terminal.

The capacitor 833 includes, over the substrate 800 having an insulatingsurface, the conductive film 812, the gate insulating film 802 over theconductive film 812, and the conductive film 819 that overlaps theconductive film 812 with the gate insulating film 802 placedtherebetween.

An insulating film 820 and an insulating film 821 are stacked in thisorder over the conductive films 814, 815, 818, and 819. A conductivefilm 822 functioning as an anode is formed over the insulating film 821.The conductive film 822 is connected to the conductive film 818 througha contact hole 823 formed in the insulating films 820 and 821.

An insulating film 824 having an opening where part of the conductivefilm 822 is exposed is provided over the insulating film 821. An ELlayer 825 and a conductive film 826 functioning as a cathode are stackedin this order over the part of the conductive film 822 and theinsulating film 824. A region where the conductive film 822, the ELlayer 825, and the conductive film 826 overlap one another correspondsto the light-emitting element 832.

In one embodiment of the present invention, the transistors 830 and 831may include a semiconductor film containing an amorphous,microcrystalline, polycrystalline, or single crystal semiconductor(e.g., silicon or germanium), or a semiconductor film containing a widebandgap semiconductor such as an oxide semiconductor.

When the semiconductor films of the transistors 830 and 831 are formedusing an amorphous, microcrystalline, polycrystalline, or single crystalsemiconductor (e.g., silicon or germanium), impurity regions functioningas source and drain terminals are formed by addition of an impurityelement imparting one conductivity to the semiconductor films. Forexample, an impurity region having n-type conductivity can be formed byaddition of phosphorus or arsenic to the semiconductor film. Further, animpurity region having p-type conductivity can be formed by addition ofboron, for instance, to the semiconductor film.

In the case where an oxide semiconductor is used for the semiconductorfilms of the transistors 830 and 831, impurity regions functioning assource and drain terminals may be formed by addition of a dopant to thesemiconductor films. The dopant can be added by ion implantation.Examples of the dopant are a rare gas such as helium, argon, and xenon;and a Group 15 element such as nitrogen, phosphorus, arsenic, andantimony. For example, when nitrogen is used as the dopant, theconcentration of nitrogen atoms in the impurity region preferably rangesfrom 5×10¹⁹/cm³ to 1×10²²/cm³.

As a silicon semiconductor, any of the following can be used, forexample, amorphous silicon formed by sputtering or vapor phase growthsuch as plasma CVD, polycrystalline silicon obtained in such a mannerthat amorphous silicon is crystallized by laser annealing or the like,and single crystal silicon obtained in such a manner that a surfaceportion of a single crystal silicon wafer is separated afterimplantation of hydrogen ions or the like into the silicon wafer.

The oxide semiconductor film includes at least one element selected fromIn, Ga, Sn, and Zn. Examples of the oxide semiconductor are an oxide offour metal elements, such as an In—Sn—Ga—Zn—O-based oxide semiconductor;oxides of three metal elements, such as an In—Ga—Zn—O-based oxidesemiconductor, an In—Sn—Zn—O-based oxide semiconductor, anIn—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxidesemiconductor, an Al—Ga—Zn—O-based oxide semiconductor, and aSn—Al—Zn—O-based oxide semiconductor; oxides of two metal elements, suchas an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxidesemiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-basedoxide semiconductor, a Sn—Mg—O-based oxide semiconductor, anIn—Mg—O-based oxide semiconductor, and an In—Ga—O-based material; andoxides of one metal element, such as an In—O-based oxide semiconductor,a Sn—O-based oxide semiconductor, and a Zn—O-based oxide semiconductor.In addition, any of the above oxide semiconductors may contain anelement other than In, Ga, Sn, and Zn, for example, SiO₂.

For example, an In—Ga—Zn—O-based oxide semiconductor refers to an oxidecontaining indium (In), gallium (Ga), and zinc (Zn), and there is nolimitation on the composition thereof.

For the oxide semiconductor film, a thin film expressed by a chemicalformula of InMO₃(ZnO)_(m), (m>0) can be used. Here, M represents one ormore metal elements selected from Zn, Ga, Al, Mn, and Co. For example, Mcan be Ga, Ga and Al, Ga and Mn, or Ga and Co.

In the case where an In—Zn—O-based material is used as an oxidesemiconductor, the atomic ratio of metal elements in a target to be usedis In:Zn=50:1 to 1:2 (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably In:Zn=20:1 to 1:1 (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio),further preferably In:Zn=15:1 to 1.5:1 (In₂O₃:ZnO=15:2 to 3:4 in a molarratio). For example, in a target used for forming an In—Zn—O-based oxidesemiconductor with an atomic ratio of In:Zn:O=X:Y:Z, the relation ofZ□□1.5X+□Y is satisfied. The mobility can be increased by keeping theratio of Zn within the above range.

Note that a purified oxide semiconductor obtained by reduction ofimpurities serving as electron donors (donors), such as moisture orhydrogen, and by reduction of oxygen defects is an i-type (intrinsic)semiconductor or a substantially i-type semiconductor. A transistorincluding the purified oxide semiconductor therefore has extremely lowoff-state current. The band gap of the oxide semiconductor is 2 eV ormore, preferably 2.5 eV or more, further preferably 3 eV or more. Withthe use of an oxide semiconductor film that is highly purified bysufficient decrease in the concentration of impurities such as moistureand hydrogen and reduction of oxygen defects, the off-state current of atransistor can be decreased.

Specifically, various experiments can prove low off-state current of atransistor in which a purified oxide semiconductor is used for asemiconductor film. For example, the off-state current of even atransistor with a channel width of 1×10⁶ μm and a channel length of 10μm can be less than or equal to the measurement limit of a semiconductorparameter analyzer, that is, less than or equal to 1×10⁻¹³ A when thevoltage between a source terminal and a drain terminal (drain voltage)ranges from 1 V to 10 V. In that case, the off-state current densitycorresponding to a value obtained by dividing the off-state current bythe channel width of the transistor is 100 zA/μm or less. In addition,the off-state current has been measured using a circuit in which acapacitor and a transistor were connected to each other and chargeflowing into or from the capacitor was controlled by the transistor. Forthe measurement, the transistor in which a channel formation region isformed in a purified oxide semiconductor film has been used, and theoff-state current density of the transistor has been measured from achange in the amount of charge of the capacitor per unit time. As aresult, it has been proven that an extremely low off-state currentdensity of several tens of yoctoamperes per micrometer (yA/μm) isobtained at a voltage between the source terminal and the drain terminalof the transistor of 3 V. Consequently, it can be understood that theoff-state current of the transistor in which the channel formationregion is formed in the purified oxide semiconductor film issignificantly lower than that of a transistor using crystalline silicon.

Unless otherwise specified, in this specification, the off-state currentof an re-channel transistor is a current that flows between a sourceterminal and a drain terminal when the potential of a gate is lower thanor equal to 0 with the potential of the source terminal as a referencepotential while the potential of the drain terminal is higher than thoseof the source terminal and the gate. Moreover, in this specification,the off-state current of a p-channel transistor is a current that flowsbetween a source terminal and a drain terminal when the potential of agate is higher than or equal to 0 with the potential of the sourceterminal as a reference potential while the potential of the drainterminal is lower than those of the source terminal and the gate.

An oxide semiconductor film can be formed, for example, by sputteringusing a target including indium (In), gallium (Ga), and zinc (Zn). Whenan In—Ga—Zn-based oxide semiconductor film is formed by sputtering, itis preferable to use an In—Ga—Zn-based oxide target having an atomicratio of In:Ga:Zn=1:1:1, 4:2:3, 3:1:2, 1:1:2, 2:1:3, or 3:1:4. When anoxide semiconductor film is formed using an In—Ga—Zn-based oxide targethaving the aforementioned atomic ratio, a polycrystal or ac-axis-aligned crystal (CAAC), which is described below, is readilyformed.

The filling rate of the target including In, Ga, and Zn is 90% or higherand 100% or lower, preferably 95% or higher and lower than 100%. Withthe use of the target with high filling rate, a dense oxidesemiconductor film is formed.

Specifically, the oxide semiconductor film may be formed as follows: thesubstrate is held in a treatment chamber with pressure reduced, asputtering gas from which hydrogen and moisture are removed isintroduced while residual moisture in the treatment chamber is removed,and the above-described target is used. The substrate temperature duringfilm formation may range from 100° C. to 600° C., preferably from 200°C. to 400° C. By forming the oxide semiconductor film while thesubstrate is heated, the concentration of impurities included in theformed oxide semiconductor film can be reduced. In addition, damage bysputtering can be reduced. In order to remove remaining moisture in thetreatment chamber, an entrapment vacuum pump is preferably used. Forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. The evacuation unit may be a turbo pump provided with acold trap. In the deposition chamber which is evacuated with thecryopump, for example, a hydrogen atom and a compound containing ahydrogen atom, such as water (H₂O) (preferably, a compound containing acarbon atom as well) are removed, whereby the impurity concentration inthe oxide semiconductor film formed in the chamber can be reduced.

Note that the oxide semiconductor film formed by sputtering or the likesometimes contains a large amount of moisture or hydrogen (including ahydroxyl group) as impurities. Moisture and hydrogen easily form a donorlevel and thus serve as impurities in the oxide semiconductor. In oneembodiment of the present invention, in order to reduce impurities suchas moisture or hydrogen in the oxide semiconductor film (in order toperform dehydration or dehydrogenation), the oxide semiconductor film issubjected to heat treatment in a reduced-pressure atmosphere, an inertgas atmosphere of nitrogen, a rare gas, or the like, an oxygen gasatmosphere, or ultra-dry air (the moisture amount is 20 ppm (−55° C. byconversion into a dew point) or less, preferably 1 ppm or less, furtherpreferably 10 ppb or less in the case where measurement is performed bya dew point meter in a cavity ring-down laser spectroscopy (CRDS)method).

By performing heat treatment on the oxide semiconductor film, moistureor hydrogen in the oxide semiconductor film can be eliminated.Specifically, heat treatment may be performed at a temperature higherthan or equal to 250° C. and lower than or equal to 750° C., preferablyhigher than or equal to 400° C. and lower than the strain point of thesubstrate. For example, heat treatment may be performed at 500° C. forapproximately 3 to 6 minutes. When an RTA method is used for the heattreatment, dehydration or dehydrogenation can be performed in a shorttime; therefore, treatment can be performed even at a temperature higherthan the strain point of a glass substrate.

Note that in some cases, the heat treatment makes oxygen released fromthe oxide semiconductor film and an oxygen defect is formed in the oxidesemiconductor film. To prevent an oxygen defect, an insulating filmincluding oxygen is used as an insulating film in contact with the oxidesemiconductor film, such as a gate insulating film, in one embodiment ofthe present invention. Then, heat treatment is performed after formationof the insulating film including oxygen, so that oxygen is supplied fromthe insulating film to the oxide semiconductor film With the abovestructure, oxygen defects serving as donors can be reduced in the oxidesemiconductor film and the stoichiometric composition of the oxidesemiconductor included in the oxide semiconductor film can be satisfied.It is preferable that the proportion of oxygen in the oxidesemiconductor film is higher than that in the stoichiometriccomposition. As a result, the oxide semiconductor film can be madesubstantially i-type and variations in electrical characteristics oftransistors due to oxygen defects can be reduced; thus, electricalcharacteristics can be improved.

The heat treatment for supplying oxygen to the oxide semiconductor filmis performed in a nitrogen atmosphere, ultra-dry air, or a rare gas(e.g., argon or helium) atmosphere preferably at temperatures rangingfrom 200° C. to 400° C., for example, from 250° C. to 350° C. It ispreferable that the water content in the gas be 20 ppm or less,preferably 1 ppm or less, further preferably 10 ppb or less.

The oxide semiconductor film is in a single crystal state, apolycrystalline (also referred to as polycrystal) state, an amorphousstate, or the like.

The oxide semiconductor film is preferably a c-axis aligned crystallineoxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystalline parts andamorphous parts are included in an amorphous phase. Note that in mostcases, the crystal part fits inside a cube whose one side is less than100 nm. From an observation image obtained with a transmission electronmicroscope (TEM), a boundary between an amorphous part and a crystalpart in the CAAC-OS film is not clear. Further, a grain boundary in theCAAC-OS film is not found with the TEM. Thus, reduction in electronmobility due to the grain boundary is suppressed in the CAAC-OS film.

In each of the crystal parts included in the CAAC-OS film, the c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic arrangement which is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. In this specification, aterm “perpendicular” includes a range from 85° to 95° unless otherwisespecified. In addition, a term “parallel” includes a range from −5° to5° unless otherwise specified.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, when crystal growth occurs from a surface side ofthe oxide semiconductor film in the process of forming the CAAC-OS film,the proportion of crystal parts in the vicinity of the surface of theoxide semiconductor film is sometimes higher than that in the vicinityof the surface over which the oxide semiconductor film is deposited.Further, when an impurity is added to the CAAC-OS film, the crystal partin a region to which the impurity is added becomes amorphous in somecases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of the surfacewhere the CAAC-OS film is formed or a normal vector of the surface ofthe CAAC-OS film, the directions of the c-axes may be different fromeach other depending on the shape of the CAAC-OS film (thecross-sectional shape of the surface over which the CAAC-OS film isdeposited or the cross-sectional shape of the surface of the CAAC-OSfilm). The crystal part is formed during deposition or by performingtreatment for crystallization such as heat treatment after deposition.

With the use of the CAAC-OS film, the change in electric characteristicsof the transistor due to irradiation with visible light or ultravioletlight can be reduced. Thus, the transistor has high reliability.

Note that part of oxygen included in the oxide semiconductor film may besubstituted with nitrogen.

The CAAC-OS film is formed by sputtering using a polycrystalline oxidesemiconductor sputtering target, for example. When ions collide with thesputtering target, a crystal region included in the sputtering targetmight be separated from the target along the a-b plane; in other words,sputtered particles having a plane parallel to the a-b plane (flatplate-like sputtered particles or pellet-like sputtered particles) mightflake off from the sputtering target. In that case, the flat plate-likesputtered particles might be able to reach a substrate while maintainingtheir shape, whereby the CAAC-OS film might be formed.

The CAAC-OS film is preferably deposited under the following conditions.

Deformation of the crystal due to impurities can be prevented byreducing the amount of impurities entering the CAAC-OS film during thedeposition, for example, by reducing the concentration of impurities(e.g., hydrogen, water, and carbon dioxide) that exist in the depositionchamber or by reducing the concentration of impurities in a depositiongas. Specifically, a deposition gas with a dew point of −80° C. orlower, preferably −100° C. or lower is used.

Increase in the substrate heating temperature during the depositionmight promote migration of sputtered particles after the sputteredparticles reach a substrate surface. Hence, the substrate heatingtemperature during the deposition is set from 100° C. to 740° C.,preferably from 200° C. to 500° C. By increasing the substrate heatingtemperature during the deposition, a flat plate-like sputtered particlewhich reaches the substrate undergoes migration on the substratesurface, so that the film of the oxide semiconductor is formed with aflat plane of the flat plate-like sputtered particle parallel to thesubstrate.

It is preferable that the proportion of oxygen in the deposition gas beincreased and the electric power be optimized in order to reduce plasmadamage at the deposition. The proportion of oxygen in the deposition gasis 30 vol % or higher, preferably 100 vol %.

As an example of the sputtering target, an In—Ga—Zn—O compound targetwill be described below.

A polycrystalline In—Ga—Zn—O compound target is made by mixing InO_(X)powder, GaO_(Y) powder, and ZnO_(Z) powder at a predetermined molarratio, applying pressure to the mixture, and then performing heattreatment on the mixture at temperatures ranging from 1000° C. to 1500°C. Note that X, Y, and Z are each a given positive number. Here, thepredetermined molar ratio of InO_(X) powder to GaO_(Y) powder andZnO_(Z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or3:1:2. The kinds of powder and the molar ratio for mixing the powder canbe determined as appropriate depending on a desired sputtering target.

Next, examples of a specific structure of a transistor included in thesemiconductor device according to the present invention will bedescribed.

A transistor illustrated in FIG. 13A is a bottom-gate transistor with achannel-etched structure.

The transistor illustrated in FIG. 13A includes a gate electrode (gate)1602 formed on an insulating surface, a gate insulating film 1603 overthe gate electrode 1602, a semiconductor film 1604 that overlaps thegate electrode 1602 with the gate insulating film 1603 placedtherebetween, and conductive films 1605 and 1606 formed over thesemiconductor film 1604. An insulating film 1607 formed over thesemiconductor film 1604 and the conductive films 1605 and 1606 may beconsidered as a component of the transistor.

The transistor in FIG. 13A may also include a backgate electrode thatoverlaps the semiconductor film 1604 with the insulating film 1607placed therebetween.

A transistor illustrated in FIG. 13B is a bottom-gate transistor with achannel protective structure.

The transistor illustrated in FIG. 13B includes a gate electrode 1612formed on an insulating surface, a gate insulating film 1613 over thegate electrode 1612, a semiconductor film 1614 that overlaps the gateelectrode 1612 with the gate insulating film 1613 placed therebetween, achannel protective film 1618 formed over the semiconductor film 1614,and conductive films 1615 and 1616 formed over the semiconductor film1614. An insulating film 1617 formed over the channel protective film1618 and the conductive films 1615 and 1616 may be considered as acomponent of the transistor.

The transistor in FIG. 13B may also include a backgate electrode thatoverlaps the semiconductor film 1614 with the insulating film 1617placed therebetween.

The channel protective film 1618 can prevent the portion serving as achannel formation region in the semiconductor film 1614 from beingdamaged in a later step (e.g., from being reduced in thickness by plasmaor an etchant in etching). Therefore, the reliability of the transistorcan be improved.

A transistor illustrated in FIG. 13C is a bottom-gate bottom-contacttransistor.

The transistor illustrated in FIG. 13C includes a gate electrode 1622formed on an insulating surface, a gate insulating film 1623 over thegate electrode 1622, conductive films 1625 and 1626 over the gateinsulating film 1623, and a semiconductor film 1624 that overlaps thegate electrode 1622 with the gate insulating film 1623 placedtherebetween and is formed over the conductive films 1625 and 1626. Aninsulating film 1627 formed over the conductive films 1625 and 1626 andthe semiconductor film 1624 may be considered as a component of thetransistor.

The transistor in FIG. 13C may also include a backgate electrode thatoverlaps the semiconductor film 1624 with the insulating film 1627placed therebetween.

A transistor illustrated in FIG. 13D is a top-gate bottom-contacttransistor.

The transistor illustrated in FIG. 13D includes conductive films 1645and 1646 formed over an insulating surface, a semiconductor film 1644formed over the insulating surface and the conductive films 1645 and1646, a gate insulating film 1643 formed over the conductive films 1645and 1646 and the semiconductor film 1644, and a gate electrode 1642 thatoverlaps the semiconductor film 1644 with the gate insulating film 1643placed therebetween. An insulating film 1647 formed over the gateelectrode 1642 may be considered as a component of the transistor.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 4

FIG. 14 illustrates an example of a panel that corresponds to oneembodiment of a display device. The panel illustrated in FIG. 14includes a substrate 700 and a pixel portion 701, a signal line drivercircuit 702, a scan line driver circuit 703, and a terminal 704 that areprovided over the substrate 700.

The pixel portion 701 includes a plurality of pixels. Each pixelincludes a display element and at least one transistor for controllingthe operation of the display element. The scan line driver circuit 703selects a pixel included in the pixel portion 701 by controlling supplyof potentials to scan lines connected to the pixels. The signal linedriver circuit 702 controls supply of an image signal to the pixelselected by the scan line driver circuit 703.

At least one of the signal line driver circuit 702 and the scan linedriver circuit 703 includes the inverter circuit described in Embodiment1 or the flip-flop circuit or the shift register circuit described inEmbodiment 2. With such a structure, the effects described in Embodiment1 or Embodiment 2 can be achieved, and the size of the pixel portion 701can be increased. Moreover, a larger number of pixels can be provided inthe pixel portion 701.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 5

The semiconductor device according to one embodiment of the presentinvention can be used for electronic devices such as display devices,personal computers, and image reproducing devices provided withrecording media (typically, devices that reproduce the content ofrecording media such as digital versatile discs (DVDs) and have displaysfor displaying the reproduced images). Other examples of electronicdevices that can include the semiconductor device according to oneembodiment of the present invention are mobile phones, game consolesincluding portable game consoles, personal digital assistants, e-bookreaders, cameras such as video cameras and digital still cameras,goggle-type displays (head mounted displays), navigation systems, audioreproducing devices (e.g., car audio systems and digital audio players),copiers, facsimiles, printers, multifunction printers, automated tellermachines (ATM), and vending machines. FIGS. 15A to 15E illustratespecific examples of these electronic devices.

FIG. 15A illustrates a portable game console including a housing 5001, ahousing 5002, a display portion 5003, a display portion 5004, amicrophone 5005, a speaker 5006, an operation key 5007, a stylus 5008,and the like. By using the semiconductor device according to oneembodiment of the present invention in a driver circuit of a portablegame console, a low-power portable game console that operates stably canbe provided. By using the semiconductor device according to oneembodiment of the present invention in the display portion 5003 or thedisplay portion 5004, a portable game console with high image qualitycan be provided. Note that although the portable game console in FIG.15A includes the two display portions 5003 and 5004, the number ofdisplay portions included in the portable game console is not limited totwo.

FIG. 15B illustrates a display device including a housing 5201, adisplay portion 5202, a support base 5203, and the like. By using thesemiconductor device according to one embodiment of the presentinvention in a driver circuit of a display device, a low-power displaydevice that operates stably can be provided. By using the semiconductordevice according to one embodiment of the present invention in thedisplay portion 5202, a display device with high image quality can beprovided. Note that a display device includes, in its category, anydisplay device for displaying information, such as display devices forpersonal computers, TV broadcast reception, and advertisement.

FIG. 15C illustrates a laptop personal computer including a housing5401, a display portion 5402, a keyboard 5403, a pointing device 5404,and the like. By using the semiconductor device according to oneembodiment of the present invention in a driver circuit of a laptoppersonal computer, a low-power laptop personal computer that operatesstably can be provided. By using the semiconductor device according toone embodiment of the present invention in the display portion 5402, alaptop personal computer with high image quality can be provided.

FIG. 15D illustrates a personal digital assistant including a firsthousing 5601, a second housing 5602, a first display portion 5603, asecond display portion 5604, a joint 5605, an operation key 5606, andthe like. The first display portion 5603 is provided in the firsthousing 5601, and the second display portion 5604 is provided in thesecond housing 5602. The first housing 5601 and the second housing 5602are connected to each other with the joint 5605, and the angle betweenthe first housing 5601 and the second housing 5602 can be changed withthe joint 5605. An image on the first display portion 5603 may beswitched depending on the angle between the first housing 5601 and thesecond housing 5602. A semiconductor display device with a positioninput function may be used as at least one of the first display portion5603 and the second display portion 5604. Note that the position inputfunction can be added by providing a touch panel in a semiconductordisplay device. Alternatively, the position input function can be addedby providing a photoelectric conversion element called a photosensor ina pixel portion of a semiconductor display device. By using thesemiconductor device according to one embodiment of the presentinvention in a driver circuit of a personal digital assistant, alow-power personal digital assistant that operates stably can beprovided. By using the semiconductor device according to one embodimentof the present invention in the first display portion 5603 or the seconddisplay portion 5604, a personal digital assistant with high imagequality can be provided.

FIG. 15E illustrates a mobile phone including a housing 5801, a displayportion 5802, an audio input portion 5803, an audio output portion 5804,operation keys 5805, a light-receiving portion 5806, and the like. Lightreceived in the light-receiving portion 5806 is converted intoelectrical signals, whereby external images can be loaded. By using thesemiconductor device according to one embodiment of the presentinvention in a driver circuit of a mobile phone, a low-power mobilephone that operates stably can be provided. By using the semiconductordevice according to one embodiment of the present invention in thedisplay portion 5802, a mobile phone with high image quality can beprovided.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

This application is based on Japanese Patent Applications serial No.2011-217150 filed with Japan Patent Office on Sep. 30, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firsttransistor; a second transistor; a third transistor; and a capacitor,wherein one of a source and a drain of the first transistor iselectrically connected to one of a source and a drain of the secondtransistor, wherein one of a source and a drain of the third transistoris electrically connected to a gate of the first transistor, wherein agate of the second transistor is electrically connected to a gate of thethird transistor, wherein a first electrode of the capacitor iselectrically connected to the one of the source and the drain of thefirst transistor, and wherein a second electrode of the capacitor iselectrically connected to the gate of the second transistor.
 2. Thesemiconductor device according to claim 1, wherein the first transistor,the second transistor and the third transistor have the sameconductivity type.
 3. The semiconductor device according to claim 1,wherein each of the first transistor, the second transistor and thethird transistor comprises an oxide semiconductor in a channel formationregion.
 4. A semiconductor device comprising: a first transistor; asecond transistor; a third transistor; and a capacitor, wherein one of asource and a drain of the first transistor is electrically connected toone of a source and a drain of the second transistor, wherein one of asource and a drain of the third transistor is electrically connected toa gate of the first transistor, wherein a gate of the second transistoris electrically connected to a gate of the third transistor, wherein afirst electrode of the capacitor is electrically connected to the one ofthe source and the drain of the first transistor, wherein a secondelectrode of the capacitor is electrically connected to the gate of thesecond transistor, and wherein the other of the source and the drain ofthe second transistor is electrically connected to the other of thesource and the drain of the third transistor.
 5. The semiconductordevice according to claim 4, wherein the first transistor, the secondtransistor and the third transistor have the same conductivity type. 6.The semiconductor device according to claim 4, wherein each of the firsttransistor, the second transistor and the third transistor comprises anoxide semiconductor in a channel formation region.
 7. A semiconductordevice comprising: a first transistor; a second transistor; a thirdtransistor; and a capacitor, wherein one of a source and a drain of thefirst transistor is electrically connected to one of a source and adrain of the second transistor, wherein one of a source and a drain ofthe third transistor is electrically connected to a gate of the firsttransistor, wherein a gate of the second transistor is electricallyconnected to a gate of the third transistor, wherein a first electrodeof the capacitor is electrically connected to the one of the source andthe drain of the first transistor, wherein a second electrode of thecapacitor is electrically connected to the gate of the secondtransistor, wherein the other of the source and the drain of the secondtransistor is electrically connected to the other of the source and thedrain of the third transistor, wherein a first potential is supplied tothe other of the source and the drain of the first transistor, wherein asecond potential is supplied to the other of the source and the drain ofthe second transistor, and wherein a signal is supplied to the secondelectrode of the capacitor.
 8. The semiconductor device according toclaim 7, wherein the signal oscillates between a first state and asecond state.
 9. The semiconductor device according to claim 7, whereinthe first transistor, the second transistor and the third transistorhave the same conductivity type.
 10. The semiconductor device accordingto claim 7, wherein each of the first transistor, the second transistorand the third transistor comprises an oxide semiconductor in a channelformation region.